Method of restoring hardenability of boron-containing steel



May 15, 1956 R. A. GRANGE ET AL 2,745,776

METHOD OF REISTORING HARDENABILTY OF BORON-CONTAINING STEEL Filed Oct. 115, 1954 United States Patent O METHOD F RESTORIN G HARDENABILITY 0F BORON-CONTAINING STEEL Raymond A. Grange, Livingston, N. J., and Ernest C. Turner, Library, Pa., assignors to United States Steel Corporation, a corporation of New Jersey Application October 13, 1954, Serial No. 461,962

2 Claims. (Cl. 14S-21.55)

This invention relates to a method of obtaining the full hardenability eiect of boron in boron-containing steels and more particularly to a method of restoring the full hardenability effect of boron in steels that have been austenitized or carburized at temperatures above about 1650 F.

The effect of a minute percentage of boron in greatly increasing the hardenability of many grades of steel has been known for many years. However, certain observers have noted that this hardenability eiect is greatly diminished when such steels are austenitized at temperatures in the vicinity of 1650 F. or higher. This is contrary to the etect obtained in nonboron-containing steels since hardenability is increased in most such steels by grain coarscning at high temperatures. This unique eiect of boron of lowering hardenability upon austenitizing is particularly undesirable in carburizing steels since in steel parts used in the carburized condition, it is common practice to quench directly from the carburizing furnace or pot in which the steel has been held at an austenitizing temperature for several hours at a temperature suciently high to diminish greatly the eiect of boron.

It is accordingly an object of the present invention to provide a treatment for boron-containing steels which will restore the full hardenability thereto after such steels have been austenitized at suiciently high temperatures to diminish the effect of boron on hardenability.

We have discovered that this object can be attained and substantially the full etfect of boron restored by holding the steel at or slow cooling to the Ara temperature or slightly above. For most steels of the type involved, this will be a temperature between 1500 and 1600 F. after austenitizing and prior to quenching. Holding at the Ars temperature for sufficient time to equalize the temperature following rapid cooling will suce. At this temperature some of the boron dissolved in the austenite by the high austenitizing temperature precipitates and thus reduces the amount of boron dissolved in austenite to the optimum amount for good hardenability. Temperatures below the Ars must be avoided because of the tendency for fern'te to form. Ordinarily this heat treatment would ice involve treating the steel after austenitizing and just before quenching by either cooling to the Ara temperature in the austenitizing furnace or by transferring the austenitized steel to a second furnace maintained at a temperature at the Ars temperature and holding it there for sufficient time to equalize the temperature at or just above the Ars temperature.

The attached Figures 1 and 2 of the drawing are illustrative of the teachings of our invention. Each of the graphs show end-quench hardenability data obtained after treating SAE-86B20 hot-rolled steel bars of 1% inch diameter. The steel contained .18% carbon, .83% manganese, .49% nickel, .49% chromium, .19% molybdenum and .0013% boron. Figure 1 shows the reduction in hardenability by holding at 17 hours for the temperatures indicated. Thus line 1 shows the effect of holding at 1650 F., line 2 at 1700 F. and line 3 at 1750" F. In Figure 2, the result of following the teaching of the invention by holding at a lower temperature is clearly shown. In this iigure, line 1 indicates the end-quench hardenability of the bars after heating at l700 F. for 20 minutes, line 3 after heating at 1750 F. for 17 hours and line 2 the effect of holding bars, previously heated at l750 F. for 17 hours, at l550 F., i. e. the Ars temperature for one hour prior to quenching. This shows that substantially the full hardenability effect of the boron is restored by holding at the lower temperature prior to quenching.

This application is a continuation-in-part of our copending application Serial No. 235,232, led July 5, 1951, now abandoned.

While we have shown a specic example of our invention, it will be understood that this example is merely for the purpose of illustration and description and that Various other forms may be devised within the scope of our invention, as dened in the appended claims.

We claim:

1. A method of restoring the hardenability eiect of boron in boron-containing steel which has been diminished by austenitizing the steel at a temperature above 1650 F. comprising holding the austenitized steel prior to quenching at the Ars temperature for suthcient time to equalize the temperature of the steel at said temperature and precipitate boron dissolved in the austenite by said austenitizing.

2. A method of obtaining the full hardenability eiect of boron in boron-containing carburized steel comprising carburizing the steel at a temperature in excess of 1650 F., equalizing the temperature of the carburized steel at the Ara temperature to precipitate excess boron dissolved in the austenite by said austenitizng and then quenching said steel.

References Cited in the le of this patent Metals Handbook, 1948 ed., pages 684 and 685. 

1. A METHOD OF RESTORING THE HARDENABILITY EFFECT OF BORON IN BORON-CONTAINING STEEL WHICH HAS BEEN DIMINISHED BY AUSTENITIZING THE STELL AT A TEMPERATURE ABOVE 1650* F. COMPRISING HOLDING THE AUSTENITIZED STEEL PRIOR TO QUENCHING AT THE AR3 TEMPERATURE FOR SUFFICIENT TIME 